January 2013

While the severe weather outbreak will be more focused on the southern states, I did this write-up just for folks in the Mid-Atlantic.

A widespread strong to severe wind event appears to be unfolding over the eastern half of the U.S. Tuesday into Wednesday, with isolated tornadoes also a threat, which is mostly across parts of the Southeast. The Mid-Atlantic has some opportunity for both on Wednesday, with the main threat obviously being a line of strong to severe winds.

We will be battling marginal lapse rates and pre-frontal clouds/rain, as we normally do in these early year setups. Very strong winds aloft (50-60 kt jet at 925mb!) suggests any sort of convection will bring down strong to severe level winds. With the strong forcing associated with the cold front, I don’t see much of a problem getting those winds to the surface. The only issue would be if a stable layer at the surface can linger ahead of the front, but that appears to be just a minor problem as the lift from the front should overcome the stable layer (if there is indeed a stable layer).

With dewpoints climbing into the lower 60s ahead of the front, we should be able to see a fairly strong frontal passage with widespread strong to severe winds. The main thing I’m watching now is the potential of a secondary low formation, which the NAM has (though that can’t be relied upon at this point). The Euro also develops a secondary low, but it comes later and further north than the NAM. If this secondary low develops before the front comes through, and if it’s far enough south, it will be able to back the winds more at the surface and would provide a greater risk for tornadoes in the Mid-Atlantic.

Somewhat higher confidence in the QPF totals brought about an expansion of the 2-4″ area. The GFS/NAM continue to suggest a snow hole in the northern VA and DC region, which remains a slight risk to the low side. Overall, I think risks are more to the high side, with localized 2-4″ totals possible within the 1-2″ contour. Likewise, the 2-4″ contour could see localized totals of 4-8″ (which is more so for the Appalachians than anywhere else).

Certainly a new breed of winter storm for this winter as we now have plenty of cold air in place, but moisture will be lacking in most areas. The good news is most of the Mid-Atlantic will see high snow ratios around 15-20:1, giving the event more bang for the buck.

Risks are more to the low side at the moment, with the current forecast leaning more towards the wetter European solutions. The NAM and GFS have been fairly insistent on a DC snow hole where accumulations are less than an inch. While entirely possible, I still think there’s enough QPF for most to get around an inch. Some of those east of the mountains in PA and NJ have a shot at 2-3″ if the higher-end QPF verifies.

Confidence is lower than normal with this storm due to the QPF issues and because the energy that will create this system is still somewhat offshore in the Pacific Northwest, so it is not getting sampled as well as it would if it was onshore.

A quick update before I head into work… shifted the forecast further south overall and tightened up the contouring in areas that have slightly higher confidence than the initial forecast.

This results in a virtual no-show north of DC aside from some flakes in the air to a dusting. Southern VA into northern NC will get rocked as the vort. max pushes through. There is a risk of this shifting even further south according to some of the hi-res models.

Here comes the first notable snow event for the southern Mid-Atlantic this winter! A powerful upper-level vort. max will push through the region tomorrow, bringing strong dynamics with it that will help create a period of moderate to heavy snowfall. The snow will start in the morning hours in the higher elevations and will work east through the afternoon and evening. The I-95 corridor will probably start off as rain in the morning. Some sleet could accompany the transition in the late morning and early afternoon, with the changeover to snow occurring around mid-afternoon on I-95 between DC and Richmond. This could lead to large traffic headaches during the afternoon rush hour.

Just in case some people might be thinking it, the current forecast is not favorable for thundersnow/thundersleet.

Very tough call on the southern edges of the contours as sleet and freezing rain make an appearance as far south as northern MD (not including the mountains).

Some locally higher totals of 4-8″ are possible, and I put a 4-8″ contour area in the spot where I think that is most likely to occur. Ratios should be above 10:1 at least at the start of the snowfall across northern PA before the warmer mid-level air tries to nose in.